The invention relates to a fixture intended to be implanted or anchored in bone tissue such as the bone tissue of the jaw, in order to support a prosthesis such as a tooth crown or the like. The fixture is provided with a surface coating of tissue-friendly material, at least in the area intended to contact the bone tissue, and has a cylindrical shaft provided with external threading and a cylindrical head. The fixture is arranged to be screwed into a previously prepared hole in the bone tissue, the diameter of the hole being substantially in equal to the core diameter of the shaft section.
A number of factors must be taken into consideration when implanting fixtures in the form of screws into bone tissue such as in the jaw-bone. When attaching a superstructure such as a dental prosthesis or the like. A primary factor is of course that integration of the fixture should occur as favourably as possible with minimum risk of infection and mechanical damage in the implantation region. Furthermore, it should be possible for the fixture to be fitted relatively quickly and simply, permitting rapid integration. Furthermore, the fixture and bandage should naturally be such as to minimize the risk of disturbance during the integration period.
One known technique, for instance, is to use a fixture comprising a cylindrically threaded shaft with a cylindrical head, the height of which is a fraction of the shaft diameter, and the diameter of which is slightly greater than the shaft diameter, with a short and gently curved transition being arranged between shaft and head. The head may be provided with a central axial hole and may be provided at its free end with formations facilitating assembly of a superstructure, for instance in the form of a crown or a base for a crown or the like.
During installation a hole is prepared in the jaw-bone, the bore diameter corresponding substantially to the core diameter of the shaft. The bore may be formed on or in the fixture itself, for instance at the lower end of the shaft. The threaded shaft of the fixture may suitably be arranged to cut the a thread in the jaw-bone for the shaft threading.
The technique described above has been used with success for many years. However, in recent years symptoms have been observed in certain cases, particularly when such fixtures are applied in the lower jaw, entailing motoric and/or sensory loss of feeling, often in the area of the lower lip. After thorough studies of these cases it has been established that the loss of feeling is caused by a nerve--the nervus alveolaris interior--located in the mandible coming into contact with the external threading of the implanted fixture. This probably causes irritation of the nerve, thereby disturbing the distal area supplied by the nerve.
It has now proved possible to eliminate these disturbances by means of the fixture proposed according to the present invention, the primary characteristic of which is that the threaded shaft section of the fixture has at least one zone without threading.